The objective of this proposal is to investigate the neurochemical basis of pain responsiveness alterations in the aged animal. In this investigation we will correlate the circadian rhythm of pain responsiveness to the circadian rhythm of brain serotonin and brain beta endorphins in young and aged animals. We will pharmacologically alter the brain concentration of serotonin and brain beta endorphin and we will study the effect of these changes on pain responsiveness in relation to aging. The results obtained from these investigations will help us to understand aging in relation to: a) pain responsiveness; b) the role of serotonin in pain responsiveness; and c) the role of beta endorphin in pain responsiveness.